1. Field of the Invention
The present invention relates to a treatment process for fluorine-containing water. The invention particularly relates to a treatment process for removal of fluorine from fluorine-containing waste water discharged from production processes for electronic parts and the like.
The industries of semiconductor manufacturing, chemical fertilizer production, ceramics, commercial aluminum, etc. use hydrogen fluoride or include production steps which generate it, and therefore the waste water contains fluorine. The present invention establishes an effective technique, for the treatment of fluorine-containing water, from the viewpoint of reinforcing environmental measures and effectively utilizing resources which has been prevalent in recent years.
2. Description of the Related Art
Standards for discharge of various chemical substances have been established in recent years through national laws on the environment, particularly on water quality, such as the Water Pollution Prevention Act and through regional and local ordinances, which have mandated that concentrations of polluting substances in waste water must be below given standard values, and total waste water treatment is being demanded. Fluorine is one substance known to upset an ecological balance when present in high concentrations in discharged waste water, and, therefore, waste water treatment facilities for fluorine removal have become very important from an industrial standpoint.
Conventional waste water treatment for fluorine removal, such as described in Japanese Unexamined Patent Publication No. 5-253576, has involved directing the fluorine-containing water into a column packed with calcium carbonate to fix the fluorine as calcium fluoride. Specifically, as shown in FIG. 1, the fluorine-containing water is directed from a water tank 1 into a column 19 packed with calcium carbonate by a conveying pump 2, and the fluorine is converted to calcium fluoride inside the column. However, due to the phenomenon of solidification of calcium compounds in the packed column and to the channeled flow of the water, i.e. a partly unbalanced formation and the establishment of water flow channels in the packed column as shown in FIG. 2, the total amount of the calcium does not react, so that a treatment malfunction often occurs whereby breakthrough is reached before the equivalent amount has reacted. In addition, fine calcium compound particles run into the treatment water, adversely affecting the instruments and later-stage treatment equipment.
Furthermore, since hydrogen fluoride is either used or generated in production steps in semiconductor manufacture, chemical fertilizer production, ceramics, commercial aluminum and other industries, they also discharge waste water containing fluorine.
Coagulating sedimentation is generally employed whereby the fluorine-containing waste water is reacted with a calcium salt such as calcium hydroxide (Ca(OH).sub.2) to produce insoluble calcium fluoride (CaF.sub.2) which fixes the fluorine, and the solid and liquid portions are then separated by gravity.
FIG. 3 shows a conventional example of this (see Japanese Unexamined Patent Publication No. 8-197070, for example). The fluorine-containing waste water is stored in a water tank 51. In a primary reaction tank 52, a calcium salt 60 such as calcium hydroxide is added to produce calcium fluoride under a pH of 6-10. An excess of the calcium salt is added to reduce the fluorine content of the waste water, and in a secondary reaction tank 53, an inorganic coagulating agent 61 comprising an aluminum, iron or other compound is added to promote the production of the insoluble calcium fluoride. An acid or alkali is added in a pH adjusting tank 54 to further promote production of the calcium fluoride at a pH of 6-8, and then an organic coagulating agent 62 comprising a partial hydrolysate of a polyacrylamide is added to a coagulating tank 55 to increase sedimentation of the product. The treated waste water is then directed to a sedimentation tank 56 and solid/liquid separation is accomplished, in the tank, by gravity.
In order to promote crystallization in the primary reaction tank 52 and secondary reaction tank 53, a method is generally employed wherein the deposited sludge 75 is returned to the primary reaction tank 52 through a conduit 13 and used as the nucleus for crystallization. A portion of the sludge separated in the sedimentation tank 56 is returned while the remainder is conveyed to a sludge reservoir 57 and dehydrated by a dehydrator 58, and the dehydrated cake 59 is treated as industrial waste.
Because the supernatant water contains insoluble calcium fluoride produced at the initial stage, it is conveyed to a later-stage treatment apparatus. The insoluble calcium fluoride which has not been deposited in the sedimentation tank 56 is removed from the storage tank 67 through a filtering column 68 packed with sand, etc. Also, for removal of the remaining fluorine in the waste water, it is treated in an adsorption column 70 packed with an adsorption agent, such as a resin spread with a metal ion such as zirconium which forms a chelate compound with fluorine ions, or active alumina. Discharge is effected after pH adjustment in a pH adjustment tank 71 in order to comply with discharge standards. In FIG. 3, 69 is a filtered water tank which functions as a filter treated water buffer and for water quality monitoring, and 72 is a storage tank which functions as a pH-adjustment treated water buffer and for water quality monitoring.
Thus, treatment of fluorine-containing waste water by conventional coagulating sedimentation requires addition of an excess of the calcium salt and addition of large quantities of coagulation agents, and this raises the cost of the treatment. In the process for improved coagulation wherein a portion of the deposited sludge in the sedimentation tank is returned to the primary reaction tank and used as the nucleus for crystallization, the coagulating agent is a factor inhibiting the crystallization. The calcium fluoride carried from the primary reaction tank reacts with the aluminum and other compounds added to the secondary reaction tank as inorganic coagulating agents, thus causing gelation and progression of crystallization. Consequently, while treatment with a calcium fluoride carrier containing no coagulating agent is more effective, addition of a coagulating agent is required because calcium fluoride is not deposited in the sedimentation tank.
In a system where the solid insoluble calcium fluoride product is separated from the liquid by gravitational sedimentation, it is impossible to avoid run-off of the insoluble calcium fluoride into the supernatant water (normally 20-50 mg/L), thus requiring later-stage filtration equipment. Sand filtration and the like are used for the later-stage filtration equipment, but this has required periodic washing operations.
Furthermore, in treatment whereby a calcium salt and coagulating agent are added to fluorine-containing waste water and the insoluble calcium fluoride salt is separated by gravity, much residual fluorine is retained in the waste water and this has required treatment in an adsorption column.
In addition, the sludge discharged from conventional treatment equipment has poor dehydrating properties because of the addition of the coagulating agent, and treatment of the dehydrated cake has necessitated much greater costs. If the calcium fluoride purity of the sludge is high it can be effectively used for purposes which offer supplementary value, for example as a material for production of hydrogen fluoride, but because of inclusion of the coagulating components in the sludge and inclusion of impurities such as SiO.sub.2 in the waste water it is impossible to achieve a high purity in the neighborhood of 95%, and therefore even if it can be effectively utilized the only options are those of low supplementary value, such as bulking agents for cements.
In the sedimentation step, solid/liquid separation is accomplished by gravitational sedimentation and therefore a large volume is necessary for sedimentation, thus requiring a large space.